Described below is a multifilament conductor with a ribbon-shaped substrate and at least one superconducting layer. The at least one superconducting layer is formed on at least one surface of the ribbon-shaped substrate and is subdivided into filaments. The ribbon-shaped substrate has a first direction parallel to its longitudinal extent and the at least one filament has a second direction parallel to its longitudinal extent. Also described below is a method for producing such a multifilament conductor.
Multifilament conductors having superconducting layers are used, inter alia, as conductors in superconducting devices. They may be used for example in superconducting windings of magnetic resonance tomographs, in motors, in generators or in current limiters. Particularly when using high-temperature superconducting (HTS) materials, for example Y2BaCu3O7 (YBCO), superconducting properties of the conductors are already achieved at liquid nitrogen temperatures. Reliable and economical superconducting devices can be produced in this way.
Second-generation (2G) industrial HTS conductors have a monocrystalline HTS thin film, in particular made of ceramic YBCO, as the current-carrying layer, which is formed on a ribbon-shaped metallic carrier. In order to apply the monocrystalline HTS thin film onto the carrier, the latter is coated with a textured multicoat buffer layer onto which the HTS layer is applied by deposition methods, for example evaporation coating, laser deposition or chemical decomposition.
On the HTS layer, a normally conducting protection or stabilization layer is additionally applied, which can electrically bridge defects and short sections in the HTS layer which have become normally conductive, and which protects the HTS layer from mechanical damage. The normally conducting layer generally is formed of silver and/or copper. The ribbon-shaped carrier, on which the layer stack of buffer, HTS and stabilization layers is applied, generally has a width in the millimeter or centimeter range.
In AC applications, a time-variant field component perpendicular to the ribbon-shaped carrier is often encountered. In the HTS layer, and to a lesser extent in the stabilization layer as well, circulating shielding currents are thereby induced which are superimposed on a transport current. These shielding currents lead to electrical losses, which are released in the form of heat and have to be dissipated from the HTS conductors by a cooling device. Economical advantages by saving energy which are achieved using HTS conductors, in comparison with known ohmic conductors, are thereby reduced or entirely negated.
Losses per length Ph/L are proportional to the alternating field amplitude ΔB, frequency f, critical current IC and effective conductor width df perpendicular to the magnetic field:Ph/L=f×ΔB×IC×df 
In NbTi and Nb3Sn superconductors, the losses are reduced by dividing the cross section into a plurality of thin filaments with a small df, which are embedded in a metal matrix, for example of copper. This measure, however, is only effective when the conductor is twisted or stranded.
An application of this principle to HTS conductors is provided by Roebel conductors. WO 03/100875 A2 discloses such a Roebel conductor, which is constructed from a plurality of parallel HTS-coated ribbon-shaped carriers. Losses in a corresponding structure of an HTS conductor are determined by the width of the individual ribbon. In order to further minimize losses, it is known for example from US 2007/0191202 A1 to subdivide the superconducting layer and the copper stabilization layer into filaments by longitudinal grooves parallel to the longitudinal direction of the ribbon-shaped carrier. Methods for forming the longitudinal grooves or trenches, extending as far as the carrier, include mechanical treatment, chemical etching, laser processing, photoresist techniques and local disruption of crystalline ordering. A filament on a carrier is thereby subdivided into a plurality of individual filaments, which extend parallel to the longitudinal axis of the carrier. The width of the individual filaments on the carrier is taken as the effective conductor width df, rather than the width of the superconducting coated carrier as a filament.
Although a reduction of the losses can be found in short conductor samples, in long conductor portions, for example in coil windings, the magnetic coupling between filaments is not however eliminated and an external alternating field, as occurs for example in coils, still induces large shielding currents. The shielding currents may exceed the critical current density of the superconducting material, so that the superconductor enters the resistive state. Significant electrical losses are incurred, which must in turn be dissipated in the form of heat.